ASTE CO (3-2) observations of the barred spiral galaxy M 83: I. Correlation between CO (3-2)/CO (1-0) ratios and star formation efficiencies

We present CO(J = 3-2) emission observations with the ASTE toward the 5′ × 5′ (or 6.6 kpc × 6.6 kpc at the distance D = 4.5 Mpc) region of the nearby barred spiral galaxy M 83. We successfully resolved the major structures, i.e., the nuclear starburst region, bar, and inner spiral arms at a resolution of 22″ (480 pc), showing a good spatial coincidence between CO and 6 cm continuum emissions. We found a global luminosity, L′_CO(3-2), of 5.1 × 10⁸ K km s^-1 pc² within the observed region. We also found L′ _CO(3-2) in a disk region (0.5 < r < 3.5 kpc) of 4.2 × 10⁸ K km s^-1 pc², indicating that (J = 3-2) emission in the disk region significantly contributes to the global L′_CO(3-2). From a comparison of CO (J = 3-2) data with CO (J = 1-0) intensities measured with the Nobeyama 45-m telescope, we found that the radial profile of the CO (J = 3-2)/CO (J = 1-0) integrated intensity ratio, R_3-2/1-0, is almost unity in the central region (r < 0.25 kpc), whereas it drops to a constant value, 0.6-0.7, in the disk region. The radial profile of star formation efficiencies (SFEs), determined from 6 cm radio continuum and CO (J = 1-0) emission, shows the same trend as that of R _3-2/1-0. At the bar-end (r ∼ 2.4 kpc), the amounts of molecular gas and the massive stars are enhanced when compared with other disk regions, whereas there is no excess of R_3-2/1-0 and SFE in that region. This means that a simple summation of the star-forming regions at the bar-end and the disk cannot reproduce the nuclear starburst of M 83, implying that the spatial variation of the dense gas fraction traced by R_3-2/1-0 governs the spatial variation of SFE in M 83.